1. Field of the Invention
The present invention relates to a method and an apparatus for operating a discharge lamp. More particularly, the present invention relates to an operating method and an operating apparatus which generate substantially linear arc discharge by utilizing acoustic resonance in a metal halide lamp. More specifically, the present invention relates to a method and an apparatus for operating a discharge lamp, capable of preventing deterioration in the transparency of an arc tube, which is otherwise caused by the reaction of the arc tube with a sealed material in the arc tube concentrated in a strip shape so as to surround the arc substantially at the center portion of the arc tube, thereby realizing a longer life of the discharge lamp.
2. Description of the Related Art
A metal halide lamp has been receiving attention as a light source for exterior illumination or interior illumination, and particularly as a light source for illuminating a commercial store, in view of its high brightness, high efficiency, excellent color rendering property, long life and the like. Recently, a smaller metal halide lamp which consumes less power has particularly attracted much attention as a light source for an imaging apparatus or a light source for automobile headlights.
In general, when this kind of discharge lamp is operated while being disposed horizontally, the arc is bent upwardly due to the influence of convection resulting from a temperature distribution generated in the arc tube. When the arc is bent, the high temperature arc of about 5000 K is disposed close to the upper side of the arc tube. As a result, a temperature in the upper side of the arc tube becomes higher than that in the lower side of the arc tube. Therefore, transparency in the upper side of the arc tube deteriorates faster than that in the lower side of the arc tube (i.e., transparency is lost), thereby adversely influencing the life of the discharge lamp. Particularly in a small low-power metal halide lamp, a distance between the arc and the arc tube becomes shorter, and thus, the bending of the arc as mentioned above has greater impact on the life of the discharge lamp.
Moreover, when the arc is bent, the shape of the arc becomes asymmetric between the upper side and the lower side thereof. As a result, in the case where the metal halide lamp is used in combination with a reflecting mirror, it is necessary to take such arc bending into consideration upon making an optical design. Consequently, optical designing becomes extremely complex and time-consuming.
As a method for eliminating such arc bending, an operating method which utilizes acoustic resonance has been proposed in Japanese Laid-open Publication No. 7-14684.
Specifically, Japanese Laid-open Publication No. 7-14684 discloses that arc bending caused by the influence of convection can be eliminated as follows. An AC current having a frequency and a waveform such that acoustic resonance in the range of about 10 kHz to about 100 kHz is excited in a radial direction of an arc tube is supplied to a discharge lamp. A frequency F.sub.V of such an AC current is selected so as to satisfy the following relationship between the frequency F.sub.V of the AC current and a frequency F.sub.R of an acoustic wave in a radial direction: EQU n.times.2F.sub.2 =m.times.F.sub.R ;
and EQU F.sub.R =3.83 C/(2.pi.R);
wherein n and m are integers; C is a sound velocity in a radial direction in the arc tube; and R is an inner diameter of the arc tube. It is described that the arc bending due to the influence of convection can be eliminated by following the above scheme.
Acoustic resonance is a phenomenon caused by the generation of a standing wave of a compression wave in the arc tube when a natural frequency of a discharge lamp, determined depending on a sealed material in the arc tube and the shape of the arc tube, becomes substantially equal to a frequency of a periodic change in electric power which is input to the discharge lamp. In general, such acoustic resonance may cause instability or extinction of arc, the bursting of arc tube, or the like. Therefore, conventionally, it has been typically attempted to avoid the generation of acoustic resonance.
In general, there are three kinds of modes in acoustic resonance, i.e., a radial direction mode, an axial direction mode, and a circumferential direction mode. The method disclosed in Japanese Laid-open Publication No. 7-14684 utilizes acoustic resonance in a radial direction among the above-mentioned three modes.
According to the study made by the inventors of the present application, however, it was confirmed that a satisfactory operating condition of a discharge lamp is not always realized even when the operating method which utilizes the acoustic resonance as disclosed in Japanese Laid-open Publication No. 7-14684 is employed.